First a high concentration of an activator emerges that marks the midline (green, see simulation on the preceding page). It is formed at the position where the competence for this is highest (dotted curve). This induces on longer range a second patterning system that marks 'left' (blue). Since the latter is locally suppressed by the midline system, it is squeezed to both sides. Due to the long-ranging competition, a full 'left'-maximum can develop only at one side. A minute asymmetry (solid curve) is sufficient to determine which side will win. After removal of the left side, first the midline marker regenerates. This triggers again the 'left' signal (blue). It is shifted now to the right
since the left side no longer exists. This simulation is in agreement with the observation . A movie
shows the concentration profiles as curves.
The left-right patterning is an obvious feature of vertebrate development but not in insects . The different mechanisms discussed above for midline formation provides a rational why a left-right pattern is easier to realize in vertebrates. Since midline elongation occurs in vertebrates by the spot-like node / organizer, a displacement of the left signal can lead to an unambiguous decision. In the following simulation for the L-R signal in vertebrate, the organizer (green) elongates the midline (red). Midline and organizer induce the 'left'- signal realized by the Nodal/Lefty2
(blue) system that becomes excluded from the midline. Due to the long-ranging inhibition, the 'left'- signal can only appear on one side of the node.